In video graphics applications, phase locked loops (PLLs) are used for generating video clocks. A main requirement of the output clock of a video PLL is low jitter. High frequency CMOS voltage controlled oscillators (VCOs) used in video PLLs have been sensitive to electrical noise on an integrated circuit's (IC's) power supply. Therefore, to maintain an adequately low amount of jitter in the video clock output, such VCO ICs have required numerous external components for power supply filtering. These external devices have added to the size and cost of the final product.
Furthermore, the output signal of a high frequency VCO circuit may require a level shifting circuit to convert the signal to one with well defined logic levels. Often it is necessary to convert signals which have a lower voltage, high logic level to signals that have a higher voltage high logic level. If the lower voltage, high logic level is well defined, invariant, and well exceeds transistor thresholds for CMOS processes, relatively simple circuits can be used to perform the level shifting. A CMOS circuit that converts TTL level logic signals up to CMOS logic levels is one example of a relatively simple level shifter.
However, if the lower voltage high logical level is not well-defined, is variable, and may not exceed a N-channel device threshold substantially, a simple TTL level shifter will often not work satisfactorily. Since the input level may not substantially exceed a device threshold, the TTL level shifter will either not function at all, or will greatly distort the duty cycle of the input signal. Also, if the input high logic level goes up in value and the frequency rises to a very high rate, a simple TTL level shifter in most cases will not be fast enough to handle the input signal. The output of a high frequency VCO is an example of a variable, not well-defined low voltage high logical level signal with varying frequencies. Therefore in such applications highly sensitive level shifting circuits capable of converting signals with varying frequencies are desired.